The invention relates to a detection device comprising a case formed by at least a first and second part able to move in translation with respect to one another between a first and second operating position. Said device comprises holding means exerting a holding force opposing movement of the first and second parts from the first operating position to the second operating position. The detection device comprises an electric contact comprising at least two electric states, closed or open, each electric state being associated with one of the operating positions of the first and second parts and securing means designed to hook said case onto an external support. The invention also relates to a seat comprising one such device.